The present invention relates to providing corrosion protection to ceramic matrix composite (CMC) material parts that contain silicon, in particular CMC material parts with a matrix constituted at least in part by silicon carbide (SiC). A particular field of application of the invention is that of parts for hot portions of gas turbines, such as combustion chamber walls, turbine rings, or turbine nozzles, for aeroengines or for industrial turbines.
For such gas turbines, the desire to improve efficiency and to reduce polluting emissions leads to envisaging ever higher temperatures in the combustion chambers.
Proposals have therefore been made to replace metal materials by CMC materials, in particular for the walls of combustion chambers or for turbine rings. CMC materials are known to possess both mechanical properties that enable them to be used for structural elements and also the ability to conserve those properties at high temperatures. CMC materials comprise fiber reinforcement made of refractory fibers, typically carbon or ceramic fibers, and densified with a ceramic matrix, typically made of SiC.
In a corrosive atmosphere (oxidizing atmosphere, in particular in the presence of moisture and/or in a saline atmosphere) and when CMC materials with an SiC matrix are used a phenomenon of the surface retreating is observed, because the silica (SiO2) that forms by oxidation on the surface of the CMC material is volatilized.
It has been recommended that an environmental barrier should be formed on the surface of the CMC material. In one such known barrier for an SiC matrix CMC material substrate, the anticorrosion function is provided by a layer made of an aluminosilicate type compound of an alkaline-earth metal, such as the compound BaO0.75.SrO0.25.Al2O3(SiO2)2, commonly known by the abbreviation BSAS. A mixed chemically barrier layer formed by a mixture of BSAS and mullite is interposed between the substrate and the anticorrosion layer in order to avoid chemical interactions between the BSAS of the anticorrosion layer and the silica formed by oxidation of the final SiC layer of the substrate. A silicon layer is formed on the substrate to enable the mixed mullite plus BSAS chemical layer to adhere. Such an environmental barrier is shown very diagrammatically in FIG. 1 and is described in particular in U.S. Pat. Nos. 6,866,897 and 6,787,195. Introducing a minority quantity of BSAS in the mixed chemical barrier layer serves to reduce significantly the sensitivity of this layer to cracking, in comparison with a chemical barrier layer formed by mullite alone. The various layers are typically formed by physical deposition, in particular by thermal plasma deposition.
Satisfactory behavior for that environmental barrier has been observed at temperatures as high as about 1200° C., but significant degradation is observed when the temperature exceeds 1300° C. It is found that at around 1310° C. chemical interaction occurs between the BSAS of the mixed chemical barrier layer and the silica formed by oxidation the silicon bonding layer, thereby leading to very rapid separation of the environmental barrier. It has also been found that the internal stresses induced by the differences in thermal behavior between the layers of the environmental barrier make the silicon bonding layer particularly sensitive to cracking.
In French patent application 06/51180, the Applicant has proposed replacing the silicon bonding layer by a layer presenting a composition gradient between pure silicon at the substrate and mullite in contact with the mixed chemical barrier layer, as shown very diagrammatically in FIG. 2. Such a composition gradient enables internal stresses of thermal origin to be accommodated, and consequently significantly reduces the sensitivity to cracking of the inner portion made of pure silicon and the outer portion made of pure mullite. The mullite can then perform its chemical barrier function effectively, in spite of being of limited thickness, and satisfactory use becomes possible above 1300° C.
Nevertheless, that leads to an increase in the number of deposits making up the environmental barrier. The fabrication process is thus lengthened. In addition, the total thickness becomes considerable, particularly since the anticorrosion layer is sensitive to a phenomenon of its surface retreating due to the silica it contains volatilizing, which phenomenon cannot be compensated other than by increasing its thickness.